The role of chromosomal proteins in maintaining the structure and regulating the function of chromatin and chromosomes in normal and neoplastic cells is being investigated. Present efforts are concentrated on determining the cellular function of two non-histone chromosomal proteins, HMG-14 and HMG-17, which may be involved in modulating the structure of transcriptionally active chromatin. We have cloned the human and chicken genes coding for HMG-14 and HMG-17. Analysis of the expression of these genes during myogenesis and erythropoiesis indicates that during differentiation, the HMG-14/-17 mRNA levels are down-regulated. This result is consistent with a role for HMG-14/-17 protein in chromatin patterning. The down-regulation is associated with distinct changes in the chromatin structure of the genes. The expression of chromosomal protein HMG-14, whose gene is located on chromosome 21, is elevated in cells and tissues of patients with Down's syndrome and in trisomy 16 mice. Aberrant expression of protein HMG-14 may have a pleiotropic effect on the expression of several genes and, therefore, may be a contributing factor to the etiology of Down's syndrome. Immunochemical analysis of the organization of the proteins in chromatin indicates that the negatively charged, C-terminal regions are exposed and available to bind to other proteins. In contrast, the central domain is tightly bound to DNA. A peptide corresponding to the DNA binding domain can act as an independent functional entity. These studies are aimed at dissecting the molecules into functional domains and at understanding the molecular mechanisms involved in the generation of transcriptionally active chromatin.